The invention relates to polyethylene terephthalate molding compositions. More particularly, the invention relates to polyethylene terephthalate molding compositions which in addition to plasticizers, nucleating agents and reinforcing agents or mixtures thereof with fillers contain molecular weight reducing agents for the polyethylene terephthalate.
Polyethylene terephthalate has been widely used for some time for fibers and films. However, the polymer has not yet achieved wide commercial acceptance for use as a molding material. Several factors have contributed to the lack of commercial acceptance of this polymer for use as a molding material. The major factors relate to the slow rate of crystallization and lack of roughness of polyethylene terephthalate.
Thus, polyethylene terephthalate has a slow rate of crystallization as compared to other moldable crystalline polymers. This has resulted in great difficulty in preparing molded articles having uniform properties and surface characteristics from this polymer using conventional molding temperatures (i.e., 70.degree.-100.degree. C.) and molding cycles. The use of higher molding temperatures (e.g., 130.degree. C. or more) and longer molding cycles have generally proven to be impractical since most conventional molds are water heated and attain temperatures of only about 110.degree. C. While there are a few oil-heated molds which attain temperatures greater than 110.degree. C., these are generally inconvenient to use and often exhibit uneven heating temperatures.
Recent approaches to the crystallization problem have involved the use of various nucleating agents to promote more rapid crystallization of the polyethylene terephthalate. A wide variety of nucleating agents have been proposed for that purpose in the prior art. Thus, U.S. Pat. No. 4,305,864 describes the use of ionizable metal salts of organic compounds having at least one acidic proton such as the sodium, potassium and lithium salts of carboxylic acids, particularly substituted carboxylic acids of which substituted benzoic acids are preferred. U.S. Pat. No. 4,284,540 refers to known nucleating agents such as benzophenone, inorganic powders and metal soaps. U.S. Pat. No. 4,172,859 refers to nucleating agents such as talc, calcium fluoride, sodium fluoride, alumina and polytetrafluoroethylene. U.S. Pat. No. 3,368,995 refers to nucleating agents which include carbon powders, carbon black, oxides of the metals of Group II of the Periodic Table, sulfates such as CaSO.sub.4 and BaSO.sub.4, phosphates such as Ca.sub.3 (PO.sub.4).sub.2, silicates such as CaSiO.sub.3 and MgSiO.sub.3, oxalates such as calcium oxalates, stearates such as magnesium stearate, benzoates such as calcium benzoates, salicylates such as zinc salicylate, talc and the neutral clays. British Patent application No. 2,015,013 refers to crystallization promoters which include the sodium or potassium salts of a hydrocarbon acid containing 7 to 25 carbon atoms.
The major approach to the problem of lack of toughness of the polyethylene terephthalate has been to utilize various reinforcing and filler materials, including a reinforcing material such as glass fibers and fillers such as mica, talc, clay and the like. Such materials are described in many of the above-mentioned patents. In addition, it has been proposed to improve the impact strength of polyethylene terephthalate by the addition thereto of a wide variety of polymeric modifiers, including among others acrylic or methacrylic monomer grafted copolymers of conjugated dienes alone or in combination with a vinyl aromatic hydrocarbon or an aromatic polycarbonate resin, see U.S. Pat. No. 4,280,948 and U.S. Pat. No. 4,280,949; copolymers of alpha-olefins and a glycidyl ester of an unsaturated aliphatic carboxylic acid, see U.S. Pat. No. 4,284,540; and the wide variety of rubbery modifiers described in U.S. Pat. No. 4,172,859.
While the polyethylene terephthalate molding compositions described in the above-mentioned patents exhibit desirable properties, in some respects they fail to exhibit the overall balance of properties needed in a fully acceptable molding material. Thus, commercially acceptable polyethylene terephthalate compositions are needed which exhibit an excellent balance of properties including high tensile strength, high flex modulus, high heat distortion temperature and good impact strength.